Exploring Quantum States of Sheets of Magnetic Dimers

The interactions that define how spins arrange themselves in a material play a fundamental role in a wide variety of physical phenomena from quantum magnetism to quantum critical phenomena to exotic superconductivity. My talk will build on one of the first problems encountered in elementary quantum mechanics - the description of a system containing two spin 1/2 identical particles - asking how a collection of these spins forms an ordered state. The Shastry-Sutherland model, which consists of a set of spin 1/2 dimers on a two-dimensional square lattice, has played an influential role in developing this general field because it is sufficiently simple to be exactly soluble, but sufficiently rich to capture the interesting physics. In this talk, I will present high-resolution x-ray and neutron scattering studies of the physical realization of the Shastry-Sutherland model, SrCu2(BO3)2, as it is tuned with pressure . The ratio of the intra and inter-dimer exchange interactions in this compound is close to a quantum critical point, where the ground state is predicted to transform from a non-magnetic singlet state to magnetic entities with only short-range correlations to a full antiferromagnet as a function of the ratio of the strength of the dimer interactions...